Minguet M, Amabilino DB, Cirujeda J, Wurst K, Mata I, Molins E, Novoa JJ, Veciana J
Institut de Ciencia de Materials de Barcelona (CSIC), Bellaterra, Spain.
Chemistry. 2000 Jul 3;6(13):2350-61.
An extensive investigation of the conformations adopted by the family of phenyl alpha-nitronyl nitroxides has been carried out. A database containing 110 crystal structures was used in a statistical study of the solid-state geometries and conformations of these radicals. This study revealed that the favoured conformations involve a twisted distortion in the imidazolyl rings and a twist between the aromatic and heterocyclic rings in the molecules. As a consequence, these radicals show two types of preferred conformations in the solid state: the pseudo-anti enantiomeric pair and the pseudo-eclipsed pair, the latter type being the most statistically probable. A new chiral member of this group of radicals that bears a lactate moiety, (R)-1, and its corresponding racemic compound, (R,S)-1, have been prepared in order to study the influence of chiral induction from the stereogenic centre on the torsion angle between the aromatic and heterocyclic rings of the alpha-nitronyl nitroxides. The X-ray crystal structures of the enantiopure and racemic compounds, which both reveal chains of molecules sustained by strong O-H...O hydrogen bonds between the carboxylic acid group and the ON group of the radical in the solid, as well as their magnetic properties have been determined. Remarkably, the molecules with a given stereogenic centre have a single helical sense between their component rings, even in the racemic crystal. Chiral induction from the stereogenic centre to the radical unit has also been proved by CD spectroscopy in the solid state. The results of these experiments have been rationalised by ab initio calculations of the spectra.
已对苯基α-硝酮基氮氧化物家族所采取的构象进行了广泛研究。在一项关于这些自由基的固态几何结构和构象的统计研究中，使用了一个包含110个晶体结构的数据库。该研究表明，有利的构象涉及咪唑基环中的扭曲畸变以及分子中芳环和杂环之间的扭曲。因此，这些自由基在固态中表现出两种类型的优选构象：伪反式对映体对和伪重叠对，后一种类型在统计学上最有可能出现。为了研究来自手性中心的手性诱导对α-硝酮基氮氧化物芳环和杂环之间扭转角的影响，制备了该自由基基团的一个带有乳酸部分的新手性成员，即(R)-1，及其相应的外消旋化合物，即(R,S)-1。已确定了对映纯和外消旋化合物的X射线晶体结构，二者均揭示了在固态中羧酸基团与自由基的ON基团之间通过强O-H...O氢键维持的分子链，以及它们的磁性。值得注意的是，具有给定手性中心的分子在其组成环之间具有单一的螺旋方向，即使在外消旋晶体中也是如此。固态CD光谱也证明了从手性中心到手性自由基单元的手性诱导。这些实验的结果已通过光谱的从头算计算得到了合理的解释。
